1. Field of the Invention
The invention relates to an MR method in which the position of an object introduced into a body to be examined is determined in order to monitor the movement of the object in the body.
2. Description of the Related Art
A method of this kind is known from Magnetic Resonance in Medicine, 29, pp. 411-415 (1993). Therein, a micro-coil mounted on a surgical instrument detects MR signals from nuclear spins situated in the vicinity of the micro-coil. During the movement of the instruments MR images are generated on the basis of gradient echos, the gradient direction being switched to a different direction after each RF pulse. The frequency of the spin resonance signals received during application of the magnetic gradient fields is linearly linked to the position of the micro-coil, so that the tip of the instrument can be localized by one-dimensional Fourier transformation of these signals.
For interventional applications it is often desirable to monitor the position of a surgical instrument, for example a catheter, during the intervention and to acquire at the same time additional anatomical information concerning the surroundings of the instrument. A first problem then encountered is that the temporal resolution of the instrument position must be very high.
A second problem consists in that the spatial position of the instrument should be determined as exactly as possible. In a method which is known from DE-A 43 10 993, an MR survey image is formed first. During the intervention, only the position of the instrument, having a suitable MR contrast, is determined. The position data thus acquired is then superposed on the data set of the MR survey image acquired before the intervention, so that the surgeon is at all times informed about the position of the instrument and the anatomy in its surroundings. However, the anatomy is liable to change in the course of the intervention, for example due to movement of the body or due to the surgical intervention, so that errors could occur upon mapping of the position in the MR survey image and in the worst case the surgeon would be shown an anatomy which is not correlated to the position of the instrument.